Hi, had the same issue after removing seat. Had to remove seat cover etc to get working for MOT last year, total ball ache. Happened again after removing seats over festive period, and after reading above comments and playing about this might help; Once seat replaced and upright, pull cord to lie flat but only tilt forward so that seat back is vertical (similar to a previous comment but with seat in situ) and pull seat belt; seems to work each time I re-fit seats now. Hope that saves someone time.
Did try your tip T6Paul which works but have to remember to release buckle and hold belt to get the seat upright again.